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POUnable to Calculate Position within Owner-Draw Text
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copied!<p>I'm trying to use Visual Studio 2012 to create a Windows Forms application that can place the caret at the current position within a owner-drawn string. However, I've been unable to find a way to accurately calculate that position.</p> <p><a href="http://www.blackbeltcoder.com/Articles/controls/mfc-hex-editor-control">I've done this successfully before in C++</a>. I've tried numerous methods in C# but have not yet been able to position the caret accurately. Originally, I tried using .NET classes to determine the correct position, but then I tried accessing the Windows API directly. In some cases, I came close, but after some time I still cannot place the caret accurately.</p> <p>I've created a small test program and posted key parts below. I've also posted the entire project <a href="http://www.softcircuits.com/Client/TestFont.zip">here</a>.</p> <p>The exact font used is not important to me; however, my application assumes a mono-spaced font. Any help is appreciated.</p> <p><strong>Form1.cs</strong> This is my main form.</p> <pre><code>public partial class Form1 : Form { private string TestString; private int AveCharWidth; private int Position; public Form1() { InitializeComponent(); TestString = "123456789012345678901234567890123456789012345678901234567890"; AveCharWidth = GetFontWidth(); Position = 0; } private void Form1_Load(object sender, EventArgs e) { Font = new Font(FontFamily.GenericMonospace, 12, FontStyle.Regular, GraphicsUnit.Pixel); } protected override void OnGotFocus(EventArgs e) { Windows.CreateCaret(Handle, (IntPtr)0, 2, (int)Font.Height); Windows.ShowCaret(Handle); UpdateCaretPosition(); base.OnGotFocus(e); } protected void UpdateCaretPosition() { Windows.SetCaretPos(Padding.Left + (Position * AveCharWidth), Padding.Top); } protected override void OnLostFocus(EventArgs e) { Windows.HideCaret(Handle); Windows.DestroyCaret(); base.OnLostFocus(e); } protected override void OnPaint(PaintEventArgs e) { e.Graphics.DrawString(TestString, Font, SystemBrushes.WindowText, new PointF(Padding.Left, Padding.Top)); } protected override bool IsInputKey(Keys keyData) { switch (keyData) { case Keys.Right: case Keys.Left: return true; } return base.IsInputKey(keyData); } protected override void OnKeyDown(KeyEventArgs e) { switch (e.KeyCode) { case Keys.Left: Position = Math.Max(Position - 1, 0); UpdateCaretPosition(); break; case Keys.Right: Position = Math.Min(Position + 1, TestString.Length); UpdateCaretPosition(); break; } base.OnKeyDown(e); } protected int GetFontWidth() { int AverageCharWidth = 0; using (var graphics = this.CreateGraphics()) { try { Windows.TEXTMETRIC tm; var hdc = graphics.GetHdc(); IntPtr hFont = this.Font.ToHfont(); IntPtr hOldFont = Windows.SelectObject(hdc, hFont); var a = Windows.GetTextMetrics(hdc, out tm); var b = Windows.SelectObject(hdc, hOldFont); var c = Windows.DeleteObject(hFont); AverageCharWidth = tm.tmAveCharWidth; } catch { } finally { graphics.ReleaseHdc(); } } return AverageCharWidth; } } </code></pre> <p><strong>Windows.cs</strong> Here are my Windows API declarations.</p> <pre><code>public static class Windows { [Serializable, StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)] public struct TEXTMETRIC { public int tmHeight; public int tmAscent; public int tmDescent; public int tmInternalLeading; public int tmExternalLeading; public int tmAveCharWidth; public int tmMaxCharWidth; public int tmWeight; public int tmOverhang; public int tmDigitizedAspectX; public int tmDigitizedAspectY; public short tmFirstChar; public short tmLastChar; public short tmDefaultChar; public short tmBreakChar; public byte tmItalic; public byte tmUnderlined; public byte tmStruckOut; public byte tmPitchAndFamily; public byte tmCharSet; } [DllImport("user32.dll")] public static extern bool CreateCaret(IntPtr hWnd, IntPtr hBitmap, int nWidth, int nHeight); [DllImport("User32.dll")] public static extern bool SetCaretPos(int x, int y); [DllImport("User32.dll")] public static extern bool DestroyCaret(); [DllImport("User32.dll")] public static extern bool ShowCaret(IntPtr hWnd); [DllImport("User32.dll")] public static extern bool HideCaret(IntPtr hWnd); [DllImport("gdi32.dll", CharSet = CharSet.Auto)] public static extern bool GetTextMetrics(IntPtr hdc, out TEXTMETRIC lptm); [DllImport("gdi32.dll")] public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj); [DllImport("GDI32.dll")] public static extern bool DeleteObject(IntPtr hObject); } </code></pre> <p><strong>Edit</strong></p> <p>The code I've posted has an issue that makes it even more inaccurate. This is a result of trying many different approaches, some more accurate than this. What I'm looking for is a fix that makes it "fully accurate", as it is in my <a href="http://www.blackbeltcoder.com/Articles/controls/mfc-hex-editor-control">MFC Hex Editor Control in C++</a>.</p>

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